Spin-Phonon Coupling Control with Strain Gradient Structures and Their Excitation by Photon

2017.11.01

Hitoshi Tabata, University of Tokyo

We have experimentally confirmed an existence of strain gradient in the ultrathin films of iron garnets. Electrical and optical anomalies have been confirmed due to the spatial inversion symmetry breaking.It has been revealed that a strain gradient, lattice strain of 5% - 0% and a tetragonal strain are introduced in samarium iron garnet (SmIG) thin films formed on GGG substrate due to a lattice mismatch of 1%. Strain gradient region is about 20 nm, where a spatial inversion symmetry is broken. The electrical permittivity is enhanced by nearly one order of magnitude, and the extinction term of the optical properties corresponding to phonon relaxation decreases. Furthermore, in the thin film in which the gradient strain region is predominant, the charge transfers from O 2p to Fe 3d levels and Fe d-d transition are observed by a magnetic circular dichroic spectroscopy. It is found that stabilization of spin alignment is increased about several times (increase pinning of energy of Bloch magnetic domain wall). On the other hand, lutetium iron garnet (LuIG) / YAG with a lattice mismatch of about 2% and/or heteroepitaxial system with almost no mismatch do not show such a lattice pinning. And the plane magnetic anisotropy is also very small. These results seem to be a key to the device design of spin and phonon excitation devices that operate stably at room temperature.